This invention relates to detection systems. In particular, but not exclusively, it relates to detection systems of the type which use light beams, such as infrared beams, in order to sense the position, or the passage of an object.
Systems using infrared or other parts of the electromagnetic spectrum for sensing the position of objects are quite common. These generally comprise one or more transmitting devices and co-operating receiving devices so that, in the simplest aspect, an infrared beam crosses a path and, if the beam is broken, the system recognizes it and therefore senses that a person or object has crossed the path. Systems of this type may be used to control entry to or from a place, for intruder alarms; or for many other purposes.
In many situations where light beams are used to sense the position of an object, it is not convenient to have the transmitting and receiving devices in line with the beam path. This may be for aesthetic, physical or environmental reasons for example. For instance, conventional access systems, known as optical turnstiles, comprise two relatively large bodies, one of them including an array of infrared transmitters and the other having an array of infrared receivers, and each being provided with electronics, control logic, etc. A person has to pass between them. Since these bodies contain electronics and detection circuitry within a metal or plastics shell, they are bulky and very visible. In environments where aesthetics are important, they are obtrusive and unwelcome.